The Heilbrun Timeline of Art History is part of the Metropolitan Museum of Art In New York. As its name suggests, it is a chronological examination of art that considers different artistic periods, geographies, and themes to present a contextualized picture of the role art has played throughout history. Essay and image work together in creating a multimedia presentation that utilizes items from the museum's collection.
Khan Academy is an online learning platform using both text and video. The information tends to the more general than the academic, but it is a useful resource in which to explore broad themes in different media and from different perspectives.
Musee de Cluny: le monde medieval
If you're in Paris, an entire museum devoted to the medieval world. But there is information available online as well. (FR / EN / SP)
The cathedrals of Europe are perhaps the most significant surviving examples of "medieval" in the world. Fortunately, many (though not as many as you would hope) have websites designed to celebrate their continuing survival:
Chartres does not have an official website, and many French cathedrals, e.g. Reims, have seen their websites embargoed by the French Ministry of Culture and can only be accessed in France at the Bibliothèque Nationale. Hard to believe, but here it is.